A transistor is an electronic semiconductor component which is used for switching and amplifying electric currents. The designation is a contraction of the designation transient resistor, which was intended to describe the transistor as a resistor controllable by current.
The field effect transistor (FET—field effective transistor) is an active nonlinear semiconductor component. In an FET the current flow (source-drain current) between source and drain is controlled by a voltage at the control electrode (gate). In individual transistors, a fourth terminal bulk (substrate) is generally connected to the source and not routed out separately. The control electrode can be a MOS structure (MOSFET—metal oxide semiconductor field effect transistor) a p-n junction (JFET—junction field effect transistor) or a Schottky contact (MESFET—metal semiconductor field effect transistor). The FET is a unipolar transistor whose current consists only of majority charge carriers. Therefore, it is unimportant to the FET whether the current flows from source to drain, or vice versa. It can therefore switch alternating currents, in contrast to bipolar transistors. The use of the various designs of field effect transistors depends primarily on the demands in respect of stability and noise behavior. In principle, there are field effect transistors for all areas of use, but MOSFETs are more likely to be used in digital technology, and JFETs and MOSFETs are more likely to be used in radio-frequency technology. FETs both on the basis of silicon and on the basis of III/V semiconductors, in particular gallium arsenide GaAs, are used for applications in radio-frequency technology. MOSFETs are used for FETs on the basis of the III/V semiconductor GaAs. Drain and source terminals are ohmic contacts, while the gate terminal is a Schottky contact. A differentiation of FETs depends on the channel type used: in the enhancement-mode FET, the conduction channel is interrupted as long as a sufficient gate voltage is not present. In the depletion-mode FET, the channel is conducting as long as it is not pinched off by a sufficiently high gate voltage. FETs are distinguished by usually lower losses than bipolar transistors. They enable very fast switching and are thus suitable for very high frequencies. FETs are distinguished by the fact that they do not have a storage type as in bipolar transistors (BJT—bipolar junction transistor). FETs can easily be connected in parallel.
FETs are generally less expensive than comparable bipolar transistors. They can be driven without power in the static case, but this leads to high charge-reversible losses at the gate.
A major advantage of FETs is the relative insensitivity to overvoltage between drain and source. When the maximum voltage is exceeded between drain and source, a so-called “breakdown” takes place. This is comparable to the Zener effect. If the quantity of energy is limited, this breakdown is reversible and the FET registered here is not destroyed.
FETs also have disadvantages. They are suitable for high voltages only to a limited extent, since losses starting from approximately 250 V are higher than in the case of an insulated gate bipolar transistor (IGBT). The parasitic diode in parallel with the drain-source path is always contained. The off switching behavior of these diodes is usually poorer than in the case of separate diodes, which often leads to undesirable oscillations. FETs are more sensitive to electrostatic discharge (ESD) at the gate than a BJT. FETs have a positive temperature coefficient (TC); the on resistance (RDS-on) is greatly temperature-dependent and rises approximately by a factor of 2 in the temperature range from 25° C. (data sheet indication) to ˜150° C. As a result, the losses and thus the heating of the component also increase.
The basic functioning of the ferroelectric field effect transistor FeFET was predicted in 1963 by Moll and Tarui [Moll, J. L.: IEEE Electronic Devices ED-10 (1963) 338]. An FeFET is a MOSFET including a ferroelectric oxide layer. The first Si-based FeFET including the ferroelectric layer Bi4Ti3O12 was realized in 1974 [Wu, S. Y.: IEEE Electronic Devices ED-21 (1974) 499].
In the case of an FeFET, the gate insulation is replaced by a ferroelectric dielectric (analogously to the floating gate in flash memories). As a result of the electrical polarization of said dielectric, the current-voltage characteristic of the source-drain junction is influenced: depending on the direction of polarization, the transistor turns off or turns on. The FeFET is written to by the application of a corresponding voltage between gate and source. The FeFET is read by the measurement of the current upon application of a voltage between source and drain. The read-out process is non-destructive.
In comparison with standard flash memories FeFETs have ferroelectric, non-volatile memories. FeFETs are distinguished by high long-term storage of what is written, and they manage with low write voltages, have a low current consumption and a high radiation hardness.
FeFETs are disadvantageous because the interface between semiconductor and ferroelectric layer has to be formed very precisely. Unfortunately, they can only have a small lattice mismatch, since otherwise uncontrolled interfacial states or traps occur. In order to avoid the interfacial states, thin slightly crystalline intermediate layers are inserted between substrate and ferroelectric layer. The ferroelectric layer must be pinhole-free.
The most common circuit variant for ferroelectric random access memories (FRAMs) is the so-called 1T1C and 2T2C cells having respectively one and two transistors and capacitors including ferroelectric dielectric. The transistor is required in any case for selecting the memory cell to be written to, since ferroelectrics do not have a sharp changeover voltage, rather the changeover probability increases with the strength of the voltage and the duration of the voltage pulse. The cells are written to by the polarization of the ferroelectric being set by a voltage pulse after the selection of the desired memory cell via the word and bit lines. The changeover between storage and erasure processes is realized in each case by a change of polarization in a ferroelectric layer.
It is advantageous that FRAMs, compared with conventional read-only memories, require no power supply for data retention. Even after the electric field has been switched off, the set state of the cell is maintained. FRAMs are furthermore compatible with conventional EEPROMs and have a practically unlimited lifetime, on account of theoretically 10 quadrillion read/write cycles, 1010 read/write cycles being guaranteed. The write time with a value of approximately 100 ns is approximately comparable to a standard SRAM.
The cell state is read out by the application of an electric field. If a change of polarization was brought about, the intensity of the current flowing through the cell changes. Since the reading method, as in the case of DRAM, has a destructive effect, that is to say that the cell content is erased, each read process is followed by a write process.
A further development of FRAMs is resistive random access memories (ReRAMs), in which it is possible to save the data situated in the RAM in the switched-off state. They are distinguished by the fact that they are non-volatile, the states are not destroyed during reading, and consume low power. ReRAMS enable a compact cell design, which in turn leads to high integration densities.
A further development of FeFETs is ferroelectric memory field effect transistors (FEMFETs), in which the gate electrode is a multilayer system consisting of an insulator, a ferroelectric and a metal. FEMFETs combine all the advantages of FeFETs. In the case of the FEMFET and also in the case of the FeFET, in comparison with the FET, just the gate electrode is replaced by a multilayer system including a ferroelectric layer. What is disadvantageous is that FEMFETs can only be processed at high temperatures. The buffer layer is very thick, since the ratio of the thickness of the ferroelectric layer to the thickness of the buffer layer is approximately 3 to 5. What is likewise disadvantageous is that generally raw earth manganites such as CeMnO3, YMnO3 or lead zirconium titanate (PZT) or barium titanate (BaTiO3), are given consideration as ferroelectric material. Strontium bismuth tantalate (SBT) can likewise be used as a ferroelectric layer. The dielectric constant ∈r of the buffer layer is ∈r˜20 to 50.
US 2005/0111252 A1 describes a field effect transistor including a source electrode, a drain electrode, a channel between the source and drain electrodes, and a gate electrode, which is separated from the channel by a continuous insulating layer or by a Schottky barrier. The channel in US 2005/0111252 A1 includes a switchable material, the conductivity of which is reversibly switchable between a state of low conductivity and a state of high conductivity. Each state of the conductive channel is persistently switchable. The field effect transistor described in US 2005/0111252 A1 has the disadvantage that the channel cannot be switched into the low conductivity state. Only the switching of the channel into the more highly conducting state by the application of an electrical voltage, injection of charge carriers, thermal and/or laser radiation is described. The switchable materials (SrZrO3, BaSrTiO3, Ta2O5, Ca2Nb2O7) are piezoelectric and not persistently switchable. The polarization state in these materials changes with the aid of electric fields, injection of charge carriers and heating. The material (SrTiO3) specified in US 2005/0111252 A1 exhibits filament formation as the switching mechanism. Filament formation is disadvantageous since it cannot be set controllably.
[Ueno, K. [et al.]: Field-effect transistor on SrTiO3 with sputtered Al2O3 gate insulator. In: Appl. Phys. Lett. Vol. 83, No. 9, Sep. 1, 2003, pages 1755-1757, S.35] uses a MISFET including an insulator (amorphous Al2O3) on a semiconducting layer (SrTiO3). The control gate consists of the insulator and a metal contact, and the voltage applied to the control gate, e.g. the gate-source voltage, controls the extent of the conducting channel in the semiconducting layer. On account of the low breakdown field strength of the amorphous Al2O3 (4 MVcm−1), the negative voltage (20 V) that can maximally be applied to the MISFET cannot be chosen to be high enough that the channel is completely pinched off by the application of a negative voltage. The conductivity of the channel is controllable only in a volatile fashion by means of the gate voltage. When no gate voltage is applied, the channel always has the conductivity value which corresponds to a gate voltage of zero volts, independently of what gate voltage was applied previously.
A very major problem is a retention time of only a few days, instead of the usual 10 years, and the occurrence of leakage currents at the control electrode or depolarization fields which reduce the polarization charge of the ferroelectric material and thus the retention time [Ma, T. P. et al.: Why is nonvolatile Ferroelectric Memory Field Effect Transistor Still Elusive. IEEE ELECTRON DEVICE LETTERS, VOL. 23, No. 7, July 2002, pages 386-389].
If analog signals are intended to be stored for a relatively long period of time and/or noise-independently (e.g. as reference or calibration signals), they are generally converted into digital signals, which have to be converted into analog values again for use in an analog circuit. The use of volatile resistive memory components which replace the conversion steps mentioned already reduces the application outlay and makes the circuit more robust.
The microphysical cause of the resistive switching processes is different for resistive memory components including different materials.
Resistive switching in resistive memory components composed of Cu-doped Ge0.3Se0.7 solid electrolytes is probably based on the electrochemical growth and on the electrochemical decomposition of metallic, filamentary paths (filament formation), which form in the solid electrolyte between an oxidizable electrode (Cu) and an inert electrode (Pt) when a voltage is applied.
Resistive memory components with filament formation can switch between two resistance states.
The performance features of resistive memory components are non-volatility, operation at low voltages and currents, a large Roff/Ron, ratio between the resistances in the “switched-off” (Roff) state and in the “switched-on” (Ron) state, fast switching times and long service lives.
Resistive switching in resistive memory components including piezo- or ferroelectric solids with metal contacts is based on the redistribution of free charge carriers at the interface between the metal contacts and the piezo- and ferroelectric solids and the shift in the position of the polarization charge when a voltage is applied.
Non-volatile resistive multilevel switching will not be possible in resistive memory components with filament formation on account of the stochastic nature of filament formation.
A neural network is constructed in a matrix-type fashion and has neurons at ends of columns and/or rows, said neurons all being connected to one another, according to the matrix, via the crossover locations, the so-called synapses.
Non-volatile analog memory elements are required for an optimum hardware realization of neural networks.
Phase change materials change their phase from crystalline to amorphous above the phase transformation temperature and are currently used primarily for latent heat storage and for data storage. The very high current densities to be used are problematic; said current densities can lead to electromigration in the metal tracks. Moreover, said phase change materials have to be thermally insulated. By way of example, the current densities for attaining the phase transformation temperature of 600° C. in GeSbTe are more than 107 A/cm2 [Lee, Benjamin C. et al.: Phase Change-Technology and the Future of Main Memory. 36th Annual International Symposium on Computer Architecture Location: Austin, Tex. 2009, IEEE MICRO 30 (2010), pages: 131-141].
An electric field forms between a statically charged boundary layer and an electrically conducting counterelectrode.
If the region between two electrically conducting electrodes is filled with a solid, liquid or gaseous material, then upon the application of an external electrical voltage the distribution of the electric field lines between the electrically conducting electrodes is determined by the dielectric properties of the solid, liquid or gaseous material.
The electric field lines begin at the positively charged electrode and end at the negatively charged electrode and illustrate the Coulomb force on electrical charges (electrons, holes, ions, charged particles) in the region between the positively and negatively charged electrodes.
If the Coulomb force is high enough, electrical charges can drift in the electric field between the electrodes as far as the oppositely charged electrode (electrons to a positively charged electrode) and on the way they can interact with other particles, ionize them or recombine with other particles.
If the distance between the electrode is too large and/or if the externally applied electrical voltage is too small, then not all the electrical charges drift in the electric field as far as the oppositely charged electrode, but rather recombine with other charged and uncharged particles.
Given different distributions of electrically active impurity atoms (donors and acceptors), semiconductor materials have intrinsic electric fields, without an electrical voltage having to be externally applied to the doped semiconductor material. Electrical charges move directionally (drift) in intrinsic electric fields.
Photocomponents, e.g. solar cells, photoelements and photodiodes, are constructed like semiconductor diodes, in principle, and do not differ fundamentally in the physical principle of action. On account of the photovoltaic effect, photogenerated charge carriers are separated in the electric field of the space charge zone of the semiconductor diode. Photodiodes and photoelements generally serve for measuring radiation, whereas solar cells generally serve for converting solar energy into electrical energy.
In order that the charge carriers that are photogenerated in the absorbent region of photocomponents contribute to the photocurrent in the semiconductor photocomponent, the photogenerated charge carriers have to reach the contact electrodes, without recombining on the way there.
If the contact electrodes are not connected, then an open-circuit voltage in the forward direction is present at the semiconductor photocomponent. With the use of only one absorber material, the open-circuit voltage cannot be greater than the bandgap of the absorber material. If the contacts are connected, then a photocurrent flows as a short-circuit current through the photocomponent.
Since the contact electrodes have to lie in direct proximity to the absorbent region of the photocomponent and since the physical principle of action of the photocomponents is based on the generation of photogenerated charge carriers, the separation thereof and transport to the contact electrodes, photocomponents having absorbent regions in which space charge zones can form on account of doping have preferably been used heretofore.
Through the use of transparent conducting contact electrodes and transparent substrate material, the shading of the absorbent region of the photocomponent by contact electrodes and substrate material can be significantly reduced.
A solar cell including only one type of absorber material can only convert the solar energy with low efficiency. Optimum conversion is effected only for the solar radiation whose energy corresponds to that of the electronic bandgap of the absorber material. A tandem solar cell combines solar cells including a plurality of absorber materials connected via tunnel barriers in a two-terminal fashion or absorber materials linked via transparent contact electrodes in a multi-terminal fashion. The photocurrent is comparable in both types of tandem solar cells and is significantly increased compared with the photocurrent of a single solar cell. The open-circuit voltage of a tandem solar cell with tunnel barriers cannot be greater than the sum of the bandgaps of all of the absorber materials used.
Ferroelectric materials, for example BiFeO3, without an external electrical voltage being applied, have ferroelectric domains with identical orientation and identical absolute value of the spontaneous polarization. Piezoelectric materials, for example BaTiO3, when an external electrical voltage is applied, have piezoelectric domains with identical orientation and identical absolute value of the spontaneous polarization. Between the domain boundaries, intrinsic electric fields form in the domains of piezo- and ferroelectric materials. The lateral extent of the domains ranges from a few nanometers up to a few micrometers to millimeters. The extent of the domain boundaries is a few nanometers.
On account of complicated domain forms, the relationship between the transport of electrical charges and the intrinsic fields in piezo- or ferroelectric domains has not been investigated very much heretofore.
Moreover, leakage currents at crystallographic defects or at domain boundaries in piezo- or ferroelectric materials are superposed on the drift current in the intrinsic fields in piezo- or ferroelectric domains.
Directional photocurrents have been observed in ferroelectric materials without an externally applied electrical voltage along the direction of intrinsic electric fields in ferroelectric domains. The open-circuit voltage depends on the lateral extent of the ferroelectric domains. The short-circuit current flows in the direction of the ferroelectric polarization.
The photovoltaic effect is fundamentally different in doped semiconductors and in ferroelectrics. The photocurrent in ferroelectrics is a few nA/cm2 in comparison with a few μA/cm2 in doped semiconductors.
The utilization of the photovoltaic effect in ferroelectrics presupposes the development of ferroelectrics having small electronic bandgaps and good volume conductivity properties.
In this regard, the photovoltaic effect in BiFeO3 with semitransparent gold electrodes upon absorption of electromagnetic waves having a wavelength of 630 nm and an irradiance of 20 mW/cm2 is 7.35 μA/cm2 [T. Choi et al.: Switchable ferroelectric diode and photovoltaic effect in BiFeO3. Science 324, 63 (2009)).
On account of the separation of photogenerated charge carriers at the domain walls having an extent of a few nanometers in BiFeO3, open-circuit voltages of 16 V and short-circuit currents of 120 μA/cm2 have been observed [S. Y. Yang et al.: Above-bandgap voltages from ferroelectric photovoltaic devices. Nature Nanotechnology 5, 143 (2010)].
The potential drop at the domain walls in BiFeO3 was determined as 10 mV and the open-circuit voltage between two contacts at the surface of BiFeO3 is determined by the intrinsic electric field in which photogenerated charge carriers can drift, without recombining. Said intrinsic electric field is approximately 7 kVcm−1 in doped semiconductors and 50 kVcm−1 in each domain wall of the ferroelectric material [S. Y. Yang et al.: Above-bandgap voltages from ferroelectric photovoltaic devices. Nature Nanotechnology 5, 143 (2010)].
It has been observed that an electrode having a contact area of a few nanometers can effectively collect photogenerated charge carriers in BiFeO3 [M. Alexe et al.: Tip-enhanced photovoltaic effects in bismuth ferrite. Nature Communications 2:256, 1-4 (2011)].
By means of a superposition of photovoltaic and piezoelectric effects, it is possible to change the volume of ferroelectric materials upon irradiation with electromagnetic waves [B. Kundys et al.: “Light-induced size changes in BiFeO3 crystals. Nature Materials 9, 803 (2010)].